## Python, <del>34</del> 25 bytes
```
lambda i:sum(zip(i,i),())
```
[Try it online!](https://tio.run/##K6gsycjPM/7/PycxNyklUSHTqrg0V6Mqs0AjUydTU0dDU/P/fwA)
Another one that returns a string instead of a list of characters:
```
for i in input():print(i,end=i)
```
[Try it online!](https://tio.run/##K6gsycjPM/7/Py2/SCFTITMPiApKSzQ0rQqKMvNKNDJ1UvNSbDM1///3SM3JyVcEAA)